Inkjet recording methods have been increasingly used these days in fields such as production of large-sized, outdoor advertisements requiring a large image area. Base materials usable for such large-sized, outdoor advertisements are tough polyvinyl chloride-based sheets that withstand outdoor use, such as sheets made of vinyl chloride resin alone or sheets made of tarpaulin, a composite material containing vinyl chloride. Some ink compositions for inkjet printing usable for printing on these polyvinyl chloride-based sheets have been proposed. For example, Patent Literatures 1 and 2 disclose photocurable ink compositions free of volatile components.
The ink compositions for inkjet printing of Patent Literatures 1 and 2 are designed to be cured using a general-type light source that emits high energy light (shorter wavelength), such as a metal halide lamp. In the case of high energy light, which has a high ability to cure ink compositions for inkjet printing, it is sufficient for the ink composition to have curability appropriate to the high curing ability.
Use of such a general-type light source, however, suffers from environmental disadvantages such as the generation of ozone and equipment disadvantages such as increased size of the radiation device and a short lamp life. Thus, a light emitting diode lamp (LED lamp, light emitted from a light emitting diode lamp is also referred as “LED light” hereinafter) that emits low energy light (longer wavelength) has recently been used as a light source for curing an ink composition for inkjet printing.
The use of LED lamps requires an ink composition for inkjet printing to have sufficient curability and printing properties (e.g., improved adhesion, reduction of cockling) under low energy light. Such improvement in photocurability generally involves reduction in the molecular weight of monomers used, which may increase the diffusibility of the ink composition into the atmosphere at lower temperatures. The ink composition is thus also required to have safety in terms of both fire defense (the flash point) and health (e.g., skin irritation).
Photocurable ink compositions for inkjet printing that can meet these requirements have been proposed.
Such photocurable ink compositions for inkjet printing include: a photocurable ink composition for inkjet printing which contains a pigment, benzyl acrylate, N-vinylcaprolactam, an acrylated amine compound containing two photopolymerizable functional groups and two amino groups in each molecule, a photoinitiator, and a sensitizer and has a flash point of 70° C. or higher (for example, see Patent Literature 3); a photocurable ink composition for inkjet printing which contains a pigment, a compound containing a (meth)acrylate group and a vinyl ether group in each molecule, a (meth)acrylated amine compound, a hindered amine compound other than (meth)acrylated amine compounds, and a photoinitiator (for example, see Patent Literature 4); a photocurable ink composition for inkjet printing which contains a pigment, a compound containing a (meth)acrylate group and a vinyl ether group in each molecule, and N-vinylcaprolactam (for example, see Patent Literature 5); and an ultraviolet curable ink composition for inkjet printing which contains a compound containing a (meth)acrylate group and a vinyl ether group in each molecule as a photopolymerizable compound and an acylphosphine oxide compound and a thioxanthone compound as photoinitiators (for example, see Patent Literature 6).
Energy saving has been promoted in various fields in recent years. In inkjet printer fields, ink jet heads driven with less energy have been developed. Further, a method of increasing the drive frequency of an ink jet head to achieve high speed printing is being developed. In order to provide a high definition recorded image without grain, an ink jet head that ejects many fine droplets is being developed.
The required properties of an ink composition for inkjet printing to use such ink jet printers include low viscosity. In an inkjet recording method, an ink composition for inkjet printing fills a nozzle and is ejected out of the nozzle by the internal pressure of the ink jet head. The time to completion of filling the nozzle with the ink composition is directly linked to the printing speed. The amount of the internal pressure required for ejection is directly linked to the driving energy.
The viscosity of an ink composition for inkjet printing greatly influences the time required for filling the nozzle and the amount of the internal pressure required. Low viscosity of an ink composition for inkjet printing results in both a short filling time and a low internal pressure. Moreover, low viscosity of an ink composition is very important for a finer nozzle, which requires longer filling time.
The photocurable ink compositions for inkjet printing of Patent Literatures 3 to 6 have relatively high viscosity, and thus they do not meet the above requirements. Although warming or heating these photocurable ink compositions for inkjet printing can reduce the viscosity without sacrificing the ink properties, such a warming or heating operation requires extra energy and damages the ink jet head.
Additionally, since the photocurable ink compositions for inkjet printing of Patent Literatures 3 to 6 contain a thioxanthone compound as a photoinitiator, curing them with low energy LED light results in yellowing of a printed material. If an amino group-containing photopolymerizable compound (e.g., N-vinylcaprolactam) as a photopolymerizable compound is used in addition to the thioxanthone compound, a printed material yellows and then fades, thus losing color balance.